It is known that a substrate may be coated with a film of material previously pulverized, and it is known that the coating may be applied to a substrate which previously may have been brought to a high temperature. According to one known technique of coating, the substrate heated to the high temperature may be moved at a constant speed passed a nozzle end and coated with product deposited on the substrate in the form of a solution. The deposited product (solution) is pyrolyzed upon contact with the heated substrate and the solvent carrier is simultaneously evaporated leaving the deposition of product in a fine layer.
The technique of spray coating has been found to present a major drawback. To this end, it has been found that the substrate is cooled by the sprayed product. In order to compensate for the degree of cooling of the heated substrate, it has been found necessary to heat the substrate to a temperature greater than the temperature at which the deposited product is normally pyrolyzed. The required additional heating is relatively costly; and the overall process technique has been found to produce a low yield.
According to another known process, product to be deposited may be sprayed on the glass substrate as a vapor. It has been found that the vapor dispersing technique also cools the substrate albeit to lesser degree than that degree of cooling during the liquid spray technique. Nevertheless, this technique suffers from the same problems and deficiencies, mentioned above. More importantly, the technique of spraying and vapor deposition are otherwise costly and complicated because of the requirement to carry out the process techniques in equipment providing at least complete fluid-tightness. Only in this manner will it be possible to prevent product which may be toxic from entering the environment.
Another known process concerns the dispersion of product in powder form on a substrate. This process benefits from the fact that the product does not substantially cool the substrate during dispersion, but the process in many applications suffers from a difficulty or inability to obtain a required very precise metering of product to be dispersed. The deficiency in the process technique is particularly evident, for example, in making special glasses coated with a very fine layer or film of metal oxide distributed on the heated glass. The metal oxide may be obtained by decomposition at high temperature, then oxidation of a compound initially in powder form. Examples of the special glasses include colored glasses, semireflecting glazings, glazings having a greenhouse effect and glazings having specific optical, thermal or electrical properties. The film generally may have a thickness of only several nanometers so not to alter the transparency of the glazing. The film, further, may consist, for example, of tin oxide resulting from the decomposition of a powder compound of the dibutyltin oxide type or the dibutyltin difluoride type.
The last-mentioned process also is considered to suffer from various problems and disadvantages. To this end, the castability of the products to be dispersed has been found to be poor resulting in a natural tendency to agglomerate, as well as a tendency while within a storage tank, to produce a vault effect or cavitation effect. Moreover, it has been found difficult or impossible to adequately meter the product to assure that the flow rate of product, in the form of a powder, has great instantaneous precision (not to exceed 1% variation) to maintain a constant thickness of film over the entire surface of the substrate. The control of thickness is necessary so as not to alter the transparency of the glazing and to obtain the properties required for special glasses. A variation in the precision of the flow, exceeding 1%, may result in irregularity in both appearance and color. These irregularities on the glazing may be readily observed. The variation may also result in insufficient and nonuniform optical, thermal and/or electrical performance also observed on the surface of the glazing.
Product in the form of powders have been metered by an Archimedes screw. However, it is difficult to obtain the necessary precision required to obtain a thickness of film that is constant over the entire surface of the substrate with the Archimedes screw. To this end, the evacuation flow of the powder may vary periodically each time that a thread of the screw pitch appears before the evacuation orifice.
As to metering systems which resort to the fluidizing of the powder, these systems also must provide a thorough fluid-tightness to prevent toxic powders, such as those of tin and/or fluoro compounds used in the treatment of glass substrates, from escaping the system. It may be difficult to obtain complete fluid-tightness, the requirements of which have been previously discussed.
Finally, powders which in theory have a good capability of being cast are known to have been metered by means of a device comprising a horizontal plate mounted to rotate about its axis and including a coaxial circular groove on its upper face. The metering device for powder may include a feeding bowl located above the plate and groove and spaced from the axis by a distance equal to the radius of the groove. A suction and distribution device operating under pressure, is located above the plate and similarly spaced from the axis.
Since both the feeding and distribution of powder are performed under pressure, the metering device is capable of functioning in a proper manner only when the powder is in an uncompacted and extremely fluid state. In addition, the metering device is capable of functioning at only very low rate of flows, near 1 kg/hr. Thus, the metering device of the prior art is not capable of use with powders having the characteristic of poor castability, or use under circumstances requiring a flow distribution on the order of 10 to 40 kg/hr. In addition, the metering device of the prior art cannot be used, since, under the action of the pressure, the powder will undergo compaction in the groove and in the suction and distribution instrumentality. Further, the metering device is adapted principally for batch mode operation. Further still, the metering device runs the risk of leading to variations in relation to the nominal flow, greater than the minimum instantaneous precision which may be tolerated.